Corrosion and oxidation resistant directionally solidified superalloy

ABSTRACT

A nickel-based superalloy having a good balance between corrosion and oxidation resistance. The alloy provides good mechanical properties. The superalloy is suited for directional solidification casting but can also be used for conventional or single crystal casting techniques. The superalloy is well suited for the hot section components such as blades, vanes and ring segments for gas turbine engines. The superalloys can be used with various thermal barrier coatings

FIELD OF THE INVENTION

The invention relates to nickel-based superalloys usable to form hotsection components of gas turbine engines.

BACKGROUND OF THE INVENTION

Nickel-based superalloys have a very good material strength at hightemperatures. These properties permit their use in components for gasturbine engines where the retention of excellent mechanical propertiesat high temperatures is required. Hot section components include vanes,rotating blades and ring segments.

The metallurgy of superalloys is a sophisticated and well developedfield. Optimization of the composition of superalloys consists ofdefining the amounts of elements which are desirably present, and theamounts of elements which are desirably absent. These impurities can insome cases be completely eliminated from the composition through thejudicious selection of melt stock material; however some elements cannotbe readily eliminated. One impurity which has long been recognized asbeing detrimental is sulfur. Sulfur was initially identified as beingdetrimental to mechanical properties, and its presence in alloycompositions was limited for that reason. However, the sulfur levelswhich do not present significant loss of mechanical properties, a bulkproperty, can in some cases still be highly detrimental to oxidationresistance, a surface property.

Oxidation resistance of superalloys is primarily due to the presence ofan adherent surface oxide scale. The composition and nature of oxidescales depends on the composition of the alloy and the environment inwhich the superalloy component operates. Several major types of oxidescales exist, which include simple as well as complex oxides/spinelsbased primarily on aluminum, cobalt, nickel, and chromium. When certainrare earth elements (i.e., those elements with consecutive atomicnumbers of 57 to 71, inclusive; also including yttrium, atomic number39) are intentionally added to the superalloy in closely controlledamounts, the oxidation resistance of components made from suchcompositions is improved. This improvement is attributed to the abilityof a rare earth element to reduce the residual sulfur content throughthe formation of sulfides and oxysulfides which stabilizes the oxidescale formed on the component surface improving the resistance of thescale and any coating thereon to spallation during use of the superalloycomponent.

The use of these superalloys at increasingly higher temperaturesrequires that a coating be applied to the superalloy component forthermal protection. The coating typically consists of applying abondcoat to the superalloy and then a thermal barrier coating (TBC) tothe bondcoat. Typical bond coats are alloys of the type MCrAlX where Mis Ni, Co, or Fe and X is commonly Y, Zr, or Hf. The bondcoat tends todegrade during prolonged high temperature exposure. The degradedbondcoat does not adequately adhere the thermal barrier coating to thesuperalloy component and spallation of the TBC occurs with complete lossof thermal protection to the component. The rate at which the bondcoatdegrades depends upon the composition of the superalloy to which it isapplied. Generally alumina forming superalloys exhibit longer bondcoatlifetimes than chromia forming superalloys. However, it is oftenpreferable to use high chromium containing superalloys for very highcorrosion resistance. The formation of an alumina scale over that of achromia scale can be enhanced by the presence of silicon.

Hence there remains a need for a superalloy with a lower propensity topromote bondcoat degradation and significantly enhance the resistance ofthe TBC to spallation.

SUMMARY OF THE INVENTION

This invention is directed to a nickel-based superalloy with a goodbalance of corrosion and oxidation resistance. The nickel-basedsuperalloy is ideally suited to directionally solidified casting, butmay also be produced by conventional casting or single crystal castingtechniques. The superalloy is well suited for applications in the gasturbine engines as hot section components such as blades, vanes, andring segments.

In one embodiment, the superalloy may be formed from materials in thefollowing percentages: 9.5 to 14.0 Cr; 7.0 to 11.0 Co; 1.0 to 2.5 Mo;3.0 to 6.0 W; 1.0 to 6.0 Ta; 3.0 to 4.0 Al; 3.0 to 5.0 Ti; 0 to 1.0 Nb;0.05 to 0.2 Hf; 0.05 to 0.2 Si; 0.005 to 0.02 B; 0 to 0.1 Zr; 0.05 to0.15 C, 0.001 to 0.1 of a mixture of two or more rare earth metalsselected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and thebalance formed from Ni. A preferred superalloy may be formed frommaterials in the following percentages: 11.6 to 12.7 Cr; 8.5 to 9.5 Co;1.65 to 2.15 Mo; 3.0 to 4.1 W; 4.8 to 5.2 Ta; 3.4 to 3.8 Al; 3.9 to 4.25Ti; 0 to 0.5 Nb; 0.1 to 0.15 Hf; 0.1 to 0.15 Si; 0.005 to 0.015 B; 0 to0.02 Zr; 0.05 to 0.11 C, 0.01 to 0.05 of a mixture of two or more rareearth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, andGd; and the balance formed from Ni. The most preferred superalloy may beformed from materials in the following percentages: 12.2 Cr; 9.0 Co; 1.9Mo; 3.8 W; 5.0 Ta; 3.6 Al; 4.1 Ti; <0.2 Nb; 0.12 Hf; 0.12 Si; 0.01 B;0.0075 Zr; 0.09 C, 0.02 of a mixture of two or more rare earth metalsselected from the group of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and thebalance formed from Ni.

An advantage of this invention is that the superalloy has goodmechanical properties and provides a unique balance between goodoxidation characteristics and good corrosion resistance.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a nickel-based superalloy with a goodbalance of corrosion and oxidation resistance. The nickel-basedsuperalloy is ideally suited to directionally solidified casting, butmay also be produced by conventional casting or single crystal castingtechniques. The superalloy is well suited for applications in the gasturbine engines as hot section components such as blades, vanes, andring segments.

The superalloy may promote a balance of corrosion and oxidationresistance suited for directionally solidified casting of hot sectiongas turbine engine components. In one embodiment, the superalloy may beformed from materials in the following percentages: 9.5 to 14.0 Cr; 7.0to 11.0 Co; 1.0 to 2.5 Mo; 3.0 to 6.0 W; 1.0 to 6.0 Ta; 3.0 to 4.0 Al;3.0 to 5.0 Ti; 0 to 1.0 Nb; 0.05 to 0.2 Hf; 0.05 to 0.2 Si; 0.005 to0.02 B; 0 to 0.1 Zr; 0.05 to 0.15 C, 0.001 to 0.1 total of at least onerare earth metals selected from the group of Y, La, Ce, Nb, Dy, Pr, Sm,and Gd; and the balance formed from Ni.

Chromium (Cr) is included to improve the alloy's high-temperaturecorrosion resistance. The reason for limiting the chromium content to9.5 to 14.0 weight percent in the present invention is to assure a goodlevel of corrosion resistance and is preferably from 11.6 to 12.7 weightpercent. The desirable high-temperature corrosion resistance cannot beensured at lower levels.

Cobalt (Co) replaces nickel (Ni) in the gamma-phase to strengthen thematrix in solid solution. Co is included in the range of 7.0 to 11.0percent by weight in the present invention to strengthen the matrix insolid solution yet have Co below the level where the proportion of thegamma prime phase is too low to have good creep strength. A preferredrange for Co is from 8.5 to 9.5 percent by weight.

Molybdenum (Mo) is a solid-solution strengthener of the gamma-phase, andcan also promote the formation of raft structure, which strengthens thesuperalloy at high temperatures. In the present invention, Mo isincluded at 1.0 to 2.5 weight percent, and is preferably 1.65 to 2.15weight percent. Mo at levels above 3.0 weight percent can be detrimentalto the creep strength and low-cycle fatigue properties of thesuperalloy.

Tungsten (W) is a solid-solution strengthener of the gamma-phase. In thepresent invention, a W content is included at 3.0 to 6.0 weight percentand is preferably 3.0 to 4.1 weight percent.

Aluminum (Al) is an element of the gamma prime phase which also forms analuminum oxide surface on the alloy to provide oxidation resistance. Inthe present invention, the Al content is 3.0 to 4.0 weight percent whichis lower than that generally required to obtain a good oxidationresistance, however, the addition of rare earth elements, silicon (Si)and Hafnium (Hf) compensates for the lower amount of Al. The preferredrange of Al is 3.4 to 3.8 weight percent.

Titanium (Ti) can replace some of the Al in the gamma prime phase toform Ni₃(Al,Ti), serving as a solid-solute strengthener of the gammaprime phase. In the present invention a Ti is included at 3.0 to 5.0weight percent and is preferably 3.9 to 4.25 weight percent.

Tantalum (Ta) is primarily in the gamma prime phase in solid solution tostrengthen the gamma prime phase and contributes to oxidationresistance. In the present invention, Ta is included at 1.0 to 6.0weight percent and is preferably at 4.8 to 5.2 weight percent where itcontributes positively to the creep strength of the superalloy.

Hafnium (Hf) improves the grain boundary strength of the superalloy. Thepresent invention includes Hf at 0.05 to 0.2 weight percent where italso promotes the formation of an alumina surface and improves theoxidation resistance of the superalloy. At higher levels of Hf themelting point of the superalloy can be diminished. The preferred rangeof Hf is 0.1 to 0.15 weight percent.

Silicon (Si) is an element that forms an oxide, SiO₂ on the surface ofthe resultant alloy which improves the oxidation resistance. Si is addedto the superalloy of the present invention at a level of 0.05 to 0.2weight percent. Si can inhibit other elements participating in the solidsolution at levels higher than 0.2 weight percent. A preferred range ofSi is 0.1 to 0.15 weight percent. The inclusion of Hf at levels similarto that of the silicon enhances the oxidation resistance provided by theSi.

Niobium (Nb) primarily partitions to and strengthens the gamma primephase. Nb acts in concert with the Ta to increase the solutionproportion of the gamma prime phase further enhancing the strengthrelative to a superalloy using Ta alone. In the present invention, Nbcan be included at a level up to 1.0 weight percent and is preferablyincluded at a level of less than 0.5 weight percent.

Carbon (C) improves the strength of grain boundaries. In the presentinvention, C is included at a range of 0.05 to 0.15 weight percent.Levels of C above this range can negatively affect the creep strength ofthe superalloy. The C is preferably included at a range of 0.05 to 0.11weight percent.

Boron (B) is also included to improve the grain boundary strength. Whenthe boron is added in excess of 0.05% the creep strength can bediminished. The content of B in the superalloy of the present inventionis limited to 0.005 to 0.02 weight percent and preferably between 0.005and 0.015 weight percent.

Zirconium (Zr) can also be included to improve the grain boundarystrength of the superalloy. In the present invention, Zr can be includedup to 0.1 weight percent and is preferably up to 0.02 weight percent.Higher levels of Zr can negatively affect the creep characteristics ofthe superalloy.

Rare earth elements Yttrium (Y), Lanthanum (La), Cerium (Ce), Gadolinium(Gd), Praseodymium (Pr), Dysprosium (Dy), Neodymium (Nd) and Erbium (Er)promote the formation of the Al₂O₃ and SiO₂ scale on the superalloy andimprove adhesive property of this protective oxide layer. The presenceof rare earth elements is believed to promote the diffusion of aluminumto the surface hence increasing the proportion of alumina in the scale.The inclusion of these rare earth elements also enhances thecompatibility of the superalloy with various coatings. An excessiveaddition of the rare earths lowers the solubility of other elements andfor this reason the combined rare earth elements are not included inexcess of 0.1 weight percent. In the present invention one or more ofthe rare earths are included in a combined range of 0.001 to 0.1 weightpercent. A preferred range for the combined rare earth elements is 0.01to 0.05 weight percent.

The presence of the rare earth elements enhances the coating life. Thisenhancement is attributed to the ability of the rare earth elements toform sulfides and oxysulfides fixing sulfur impurities which preventstheir diffusion to the surface and degrades the alumina scale on thesuperalloy.

As indicated above, a preferred superalloy for high corrosion resistanceand an improved oxidation resistance may be formed from materials in thefollowing percentages: 11.6 to 12.7 Cr; 8.5 to 9.5 Co; 1.65 to 2.15 Mo;3.0 to 4.1 W; 4.8 to 5.2 Ta; 3.4 to 3.8 Al; 3.9 to 4.25 Ti; 0 to 0.5 Nb;0.1 to 0.15 Hf; 0.1 to 0.15 Si; 0.005 to 0.015 B; 0 to 0.02 Zr; 0.05 to0.11 C, 0.01 to 0.05 of one or more rare earth metals selected from thegroup of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed fromNi. A most preferred superalloy composition may be formed from materialsin the following percentages: 12.2 Cr; 9.0 Co; 1.9 Mo; 3.8 W; 5.0 Ta;3.6 Al; 4.1 Ti; <0.2 Nb; 0.12 Hf; 0.12 Si; 0.01 B; 0.0075 Zr; 0.09 C,0.02 of a mixture of one or more rare earth metals selected from thegroup of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; and the balance formed fromNi.

The superalloy of the present invention is ideally suited fordirectionally solidification casting. However, it can be readilyproduced by conventional casting or single crystal casting techniques.The superalloy is well suited for the hot section components such asblades, vanes and ring segments for gas turbine engines. The superalloyscan be used with various thermal barrier coatings.

Alternatives for the alloy composition and other variations within therange provided will be apparent to those skilled in the art. Variationsand modifications can be made without departing from the scope andspirit of the invention as defined by the following claims.

1. A nickel-based superalloy expressed in weight percentages consistingessentially of: 9.5 to 14.0 Cr; 7.0 to 11.0 Co; 1.0 to 2.5 Mo; 3.0 to6.0 W; 1.0 to 6.0 Ta; 3.0 to 4.0 Al; 3.0 to 5.0 Ti; 0 to 1.0 Nb; 0.05 to0.2 Hf; 0.05 to 0.2 Si; 0.005 to 0.02 B; 0 to 0.1 Zr; 0.05 to 0.15 C;0.001 to 0.1 of a mixture of two or more rare earth metals selected fromthe group of consisting of Y, La, Ce, Nb, Dy, Pr, Sm, and Gd; andbalance formed from Ni.
 2. The superalloy of claim 1, wherein thesuperalloy consisting essentially of: 11.6 to 12.7 Cr; 8.5 to 9.5 Co;1.65 to 2.15 Mo; 3.0 to 4.1 W; 4.8 to 5.2 Ta; 3.4 to 3.8 A1; 3.9 to 4.25Ti; 0 to 0.5 Nb; 0.1 to 0.15 Hf; 0.1 to 0.15 Si; 0.005 to 0.015 B; 0 to0.02 Zr; 0.05 to 0.11 C; 0.01 to 0.05 of a mixture of two or more rareearth metals selected from the group of consisting of Y, La, Ce, Nb, Dy,Pr, Sm, and Gd; and the balance formed from Ni.
 3. The superalloy ofclaim 1, wherein the superalloy consisting essentially of: 12.2 Cr; 9.0Co; 1.9 Mo; 3.8 W; 5.0 Ta; 3.6 Al; 4.1 Ti; <0.2 Nb; 0.12 Hf; 0.12 Si;0.01 B; 0.0075 Zr; 0.09 C; 0.02 of a mixture of two or more rare earthmetals selected from the group consisting of Y, La, Ce, Nb, Dy, Pr, Sm,and Gd; and the balance formed from Ni.